Process for air separation by liquefaction and rectification



Nov. 8, 1960 M. GRENIER PROCESS FOR AIR SEPARATION BY LIQUEFACTION AND RECTIFICATION 2 Sheets-Sheet 1 Filed Sept. 5, 1957 //vvE/vrok Me ue/cz Gleam 1e Nov. 8, 1960 GRENlER 2,959,021

PROCESS FOR AIR SEPARATION BY LIQUEFACTION AND RECTIFICATION Filed Sept. 5, 1957 2 Sheets-Sheet 2 Unite States Patent PROCESS FOR AIR SEPARATION BY LIQUEFAC- TION AND RECTIFICATION Maurice Grenier, Paris, France, asn'gnor to LAir Liquide Societe Anonyme pour lEtudes et lExploitation des Procedes Georges Claude, Paris, France Filed Sept. 5, 1957, Ser. No. 682,151

Claims priority, application France Sept. 25, 1956 12 Claims. (Cl. 62-29) The present invention relates to a process for separating air into its elements by liquefaction and rectification,- in which oxygen is taken off, at least partly, in a liquid condition, then at least partly warmed up in counter current to a first air fraction under pressure, which'is then expanded and partly liquefied, while the remaining elements of air are taken off in a gaseous condition and warmed up in counter-current to a second air fraction under pressure which is then introduced at a constant cold content into the rectification apparatus.

It is known that, in air rectification apparatus in which oxygen is extracted in a liquid condition, then vaporized by heat exchange with compressed air, which, due to this fact, is at least partly liquefied, there often occurs a difiiculty, owning to the fact that the demand for gaseous oxygen is not constant, and, consequently, the important the production of cold on the air circuit cooled in counter current to the nitrogen, for instance by varying the proportion of that air expanded with externalwork. Thus it is not possible to avoid disturbing the operation of the apparatus.

It has been proposed already to regularize the contribution in cold by keeping constant the amount of air subjected to the expansion, and by liquefying by heat exchange with the fraction of liquefied air some nitrogen taken from a rectification column and compressed; the air is then introduced into the rectification apparatus at its dew point, and a liquid nitrogen storage tank makw it possible either to absorb a temporary excess in production thereof, or, on the contrary, to supply a certain amount of it for supplying the rectification apparatus and keep the total amount of cold supplied constant. The does not prevent, however, the disturbing, we certain extent, of the operation of the rectification apparatus, as the amount of gaseous nitrogen taken therefrom to to be liquefied by heat exchange with the incoming air remains variable.

The process of the invention consists in that the unliquefied portion of the first fraction of air under pressure is sent for heat exchange with said first air fraction, then is evacuated while the liquefied portion of said first air fraction is introduced alone and at a-constant rate into the rectification apparatus. Two circuits are thus maintained for supplying, independently,tthe rectification apparatus, both at a constant rate:

. (a) Liquid air, obtained by heat exchange with part, at least, of the oxygen;

(b) Air, mostly gaseous, cooled by heat exchange with the nitrogen and possibly with part of the oxygen.

The air of the first fraction being liquefied at a variable rate, varying with the rate of flow of oxygen which is vaporized simultaneously, it is convenient, in practice, to interpose, before the rectification apparatus liquid air 2,959,021 Patented Nov. 8,

. pressures, and'in mutual heat exchange. The liquid air,

sent at a constant rate to the rectification apparatus will be introduced, preferably, in the column at the lowest pressure. It is in such a case, indeed, that the cold amount brought in by a given mass of liquid air is largest;

It should be noted that it is possible not to send the whole of the liquid air produced to the storage means which feed the rectification apparatus; a small portion thereof may, for instance, be sent to an auxiliary device, such as a washing device for gaseous air, already cooled, intended for completing the purification thereof.

The liquid oxygen taken from the rectification apparatus is preferably sent also to storage means, from which it is taken to be warmed up, possibly after compression in'a liquid condition.

1 Such storage means makes it possible to effect the takeofisfat variable rates, or even in a discontinuous manher, without disturbing the operation of the rectification columns.

, With a view to compensating, to some extent the fluctuations in the cold' contribution to the first fraction .of compressed air by the warming up and possibly the vaporization of oxygen, while ensuring for the air liquefyir'ig device an operation as regular as possible, one operates preferably as follows, according to the invention.

Air is compressed at a constant rate, and a variable proportion, which constitutes the above mentioned first frac tion, is sent for heat exchange with the oxygen to be;

warmed up; the additional compressed air is expanded I with external work, then heated by heat exchange with this same first air fraction and evacuated. In this manner, when little oxygen is vaporized, an important amount of cold is produced by expansion with external work of a large portion of the air treated, and the amount of air subjected to liquefaction is relatively small. On the other hand, when vaporizing a large amount of oxygen, the cold contribution by expansion is decreased or cut-off, and the cold is transmitted to a more important amount of air.

A specially important use of the invention relates tothe case when the plant for the vaporization of oxygen .is-

very distant from the rectification plant. The plant, of course, may not be unique and in the most frequent case, there corresponds to a rectification plant of a relatively large capacity, a number of vaporization plants of lesser.

capacity separated from one another. It is advantageous for a user of oxygen, to have available an apparatus for the vaporization thereof either when he uses important amounts of gaseous oxygen under a low pressure at a variable rate or even in a discontinuous manner, for instance for metallurgical operations, or when he desires pressure, of a known type.

does not require the use of metal cylinders which'consti tute an important dead weight, much larger than that'of the oxygen. On the other hand, the amount of cold gen erated by the vaporization of oxygen was generally 'lo's't' heretofore, as vaporization was effected by heating up in According to one mode of application of the invention,

the rectification plant on the one hand, and the oxygen vaporizing plant, assumed "to be unique, on thefoth e'r storage means which may be fed at a variable rate ,or 2' hand, are both provided with liquid-air and liquid oxygen storage means. The recuperation of the heat of vaporization of the liquid oxygen then takes place as follows:

The rectification plant supplies, at a constant rate, the adjacent storage means; liquid oxygen is sent, therefrom, to the storage means of the vaporizing plant by a known conveying means, for instance tank trucks or through a pipe. The oxygen is vaporized at a variable rate by heat exchange with compressed air which is liquefied also at a variable rate and which is stored in storage means. The same conveying means, when returning, brings liquid air taken from that store to the store adjacent to the rectifioation apparatus, the latter storage means feeding the installation at a constant rate. 1

Thus one benefits by the ease of transportation of liquid air and liquid oxygen, while sacrificing only the power expenditure corresponding to the obtention of gaseous oxygen. In addition, the plants of rectification on the one hand, and of oxygen vaporizing and air liquefyingon the other hand, are made independent of each other, so that one may without any inconvenience, stop one of themmomentarily without hindering the operation of the other one,. as long as the storage means which supplies the latter is not exhausted. It is possible, for instance,. to have the rectification plant operate intermittently only, during periods in the day when power is available at a lower cost, for instance at night. In addition, one may proceed Without any drawback with the necessary periodic frost removals.

Theer are described hereinafter, with reference to the appended drawing, and only by way of example, two forms of embodiment of the present invention.

Figure 1 represents a plant for the separation of air into its elements by rectification supplying the entiretyof the oxygen in a liquid condition, with, on the one hand, a

supply of gaseous, compressed air, cooled by heat exchange with nitrogen, and, on the other hand, a supply of liquid air from a tank supplied from an installation for warming up oxygen under a relatively high pressure.

Figure 2 represents a plant for the vaporization of oxygen under a moderate pressure, with a concomitant air liquefaction.

In the rectification plant represented in Figure l, the air is brought at a constant rate through conduit 1 to the turbo-compressor 2 which raises it to a relatively low pressure, of the order, for instance, of kg./cm. abso lute. The compressed air is sent through conduit 3 to one of the exchangers-regenerators of one pair 4A-4B which ensures, simultaneously its cooling and its purification by depositing of water and carbon dioxide. When the air passes through one of the regenerators, there passes through the other one cold gaseous nitrogen from the rectification plant. The connections of these regenerators with the air and nitrogen conduits are interchanged at fixed time intervals, in accordance with a known method, by means of a set of valves.

Upon issuing from the regenerator, the air is sent, through the conduit 5 to the rectification plant 6. The latter comprises a high pressure column 7 and a low' pressure column, 8, in which the pressure is closeto theatmospheric pressure. These two columns are in mutual heat exchange relation by means of the condenser vaporizer 9. In the high pressure column 7, the air is sep-- arated into a liquid enriched in oxygen, gathered at the bottom of the column, and impure nitrogen which condenses in the condenser 9 and is collected in the troughs 9A. The air enriched in oxygen is tapped through the conduit 10 and sent to the exchanger-eoolerll where it is cooled. by the gaseous nitrogen from the top. of. the lowpressurecolumn; then it is introduced through therexpan sion valve 12 into that column at 12A.

The impure liquid nitrogen gathered in the troughs 9A; is sent through the conduit 13 to the exchanger cooler 14 where it is cooled by the gaseous nitrogen from the top of the low pressure column. It is then introduced; through the expansion: valve 15 and 15Arinto'thetop of.

called liquefier.

that column, where it plays the part of reflux liquid. The low pressure column is further supplied with liquid air accumulated in the tank 16 which receives, at a variable rate, liquid air produced in the adjacent oxygen vaporizing plant. The liquid air is tapped from the tank through the conduit 17 and introduced at a constant rate controlled by the valve 18 at 18A into the low pressure column. The air is separated, in the low pressure column, into pure oxygen, which is collected at the bottom thereof, around the tubes of the condenser-vaporizer 9 and into pure gaseous nitrogen, which is released at the top of the column through the conduit 22,

The pure liquid oxygen is sent, through the valve 19 and conduit 19A to the tank 20, where it accumulates. The gaseous oxygen which is released in that tank is brought back through the conduit 21 to the low pressure rectification column.

The gaseous nitrogen released at the top of the low pressure column is sent, through conduit 22 to the coolers 14 and 11, where it cools successively the impure liquid nitrogen, and the liquid enriched with oxygen from the high pressure column. Then through the conduit 32, it goes to one of the exchangers-regenerators 4A-4B already described, where it heats up and vaporises the impurities of the incoming air. It is evacuated through conduit 24.

The liquid oxygen accumulated in the tankztl is evacuated as follows, according to the use intended for it. If it is desired to evacuate it in a liquid condition, for instance for loading vehicles for its transportation to long. distances, it is extracted directly through the valve 25. If, on the other hand, it is desired to heat it on the spot, for instance under a high pressure close to kg./cm. with a view to the filling of storing cylinders, it is sent, through the valve 26 and conduit 27 to the pump 28, which raises its pressure to the desired value. Then through conduit 29 it goes to the exchanger 30, the so- In the latter, the oxygen is vaporised by heat exchange with cold air circulating in countercurrent, which it allows to be liquefied. Then it goes to the exchanger 31, the so-called hot exchanger, where it is warmed up substantially to room temperature by a heat exchange with compressed air. It is finally sent through the conduit 32 to the device for its utilization.

The air which it is desired to liquefy at least partly for feeding the tank 16 associated with the rectification plant, is sent at a constant rate through the conduit 37 to the compressor 38 which raises it to a high pressure. higher than that it is desired to obtain for oxygen, for instance 200 kg./cm. It is purified in a known manner, in the purifying device 39, then cooled in the hot exchanger 31, in counter-current, either of vaporized oxygen or of expanded air as indicated hereinafter or of both. It is evacuated therefrom through the conduit 40 and separated into two portions, in proportions which are adjustable according to the cold contribution from the vaporization of'oxygen.

The first portion is sent through the valve 41 to the liquefier exchanger, 30, where it is cooled by heat exchange with liquid oxygen and the cold expanded air, already mentioned. Then through conduit 42 it goes to the cooling exchanger 43, where it is cooled by the gaseous air evolved in the tank 16, then sent through conduit 44 and the expansion valve 45 to that same tank. The gaseous air evolved in that tank is sent, through the conduit 48 to the cooling exchanger 43 then united through the conduit 49 with the expanded and cold air at the entrance to the liquefier exchanger 30.

The second portion of the compressed air, intended'for supplying the necessary amount of cold by expansion with external work when the cold supplied by the vaporization of oxygen is insufiicient, is sent, in an amount whichcan be controlled by the valve 50 and the conduit 51 to the expansion machine 52 then through the con duit:53 tothe liquefier-exchanger 30,- after addition-eithev.3 cold air from the tank 16 brought through the conduit 49. It then passes through the hot exchanger 31 then is evacuated substantially at room temperature through the conduit 54.

The plant for vaporizing oxygen at a moderate pressure represented in Figure 2 offers the same overall arrangement as the device for heating the oxygen with a concomitant cooling of air, which constituted a part of the plant just described. It has been modified a little, however, for responding more easily to the very important variations of the rates of oxygen to be vaporized and air to be liquefied; it comprises two liquid oxygen pumps instead of one, and two air expansion turbines instead of one alternating expansion machine.

The tank for liquid oxygen 20 is supplied by means of transport vehicles loaded with liquid oxygen at a rectification plant and which discharge that oxygen, after the valve 19 has been opened, into the conduit 19A. These same transport vehicles are then loaded with the liquid air accumulated in the tank for storing it, through the conduit 17, after the valve 18 has been opened, then bring this liquid air back to a storage tank adjacent to a rectification plant.

According to the rates of oxygen to be vaporized the oxygen vaporizing plant operates in one of the following manners, the rate of flow of the compressed air introduced into the installation remaining constant.

(a) The amount of oxygen to be vaporized is zero. The two pumps 28A and 28B are then stopped and the valves 26A and 26B are closed. The two turbines 52A and 52B are in operation, the valves 50A and 50B being open. The opening of the valves 41, 50A and 50B is so adjusted that a large portion of the compressed air which comes in goes through the expansion turbines and gives a sufficient production of cold for making it possible to liquefy the other portion;

(11) The amount of oxygen to be vaporized is medium. A single one of the liquid oxygen pumps, 28A for instance, is in operation, the corresponding valve 26A being open. A single one of the expansion turbines 52A for instance, is in operation, the valve 50A being open and the valve 50B closed. A more important proportion of the incoming compressed air goes through the valve 41 and liquefies in the liquefier-exchanger 30;

(c) The amount of oxygen to be vaporized is large. The two pumps 28A and 28B are in operation, the valves 26A and 26B being open. The two expansion turbines 52A and 52B are stopped, and the two valves 50A and 50B are stopped. The entirety of the incoming air goes through the valve 41 and is liquefied in the liquefier exchanger 30.

What I claim is:

I. In a method for producing oxygen by the liquefaction and rectification of air and supplying it to a variable demand, wherein oxygen is taken off, at least in part, in the liquid state, then at least partly warmed up countercurrently to a first air fraction under pressure, which is thereafter expanded and partly liquefied, while the remaining products of the rectification are taken oif in the gaseous state and warmed up countercurrently to a second air fraction under pressure, which is thereafter introduced at a constant rate and a constant cold content into the rectification zone, the improvement of warming up the unliquefied part of the first air fraction by heat exchange with said first air fraction, and thereafter evacuating it, and introducing the liquefied part of said first air fraction at a constant rate into the rectification zone.

2. A method for producing oxygen according to a variable demand by the liquefaction and rectification of air, comprising the steps of: taking off at least part of the oxygen product of rectification in a liquid condition and at least partly warming it up in heat exchange with a first air fraction under pressure, taking ofi in a gaseous condition the remaining products of the rectification and warming them up in heat exchange with a second air fraci tion under pressure, expanding said first air fraction and partly liquefying it, warming up the unliquefied part thereof by heat exchange with said first air fraction, and thereafter evacuating it, feeding the liquefied part thereof to a store of liquid air, then supplying it to the rectification zone at a constant rate, and introducing said second air fraction at a constant rate and a constant cold contet into the rectification zone.

3. A method for producing oxygen by the liquefaction and rectification of air in at least two rectifying zonesunder successively lower pressures and supplying it to a variable demand, comprising the steps of: taking off the oxygen product, at least in part, in the liquid condition from a rectification zone, then at least partly warming it up by heat exchange with a first air fraction, which is thereafter expanded and partly liquefied, taking oif the remaining products of the rectification in the gaseous condition and warming them up by heat exchange with a second air fraction under pressure, which is thereafter introduced at a constant rate and a constant cold content into at least a rectifying zone, sending the unliquefied part of the first air fraction into heat exchange with said first air fraction, and thereafter evacuating it, and introducing the liquefied part of said first air fraction at a constant rate into the rectifying zone under the lowest pressure.

4. A method for producing oxygen according to variable demand by the liquefaction and rectification of air, comprising the steps of: taking off at least part of the oxygen product in the liquid state and feeding it to 'a store of liquid oxygen, taking it from said store and at least partly warming it up by heat exchange with a first air fraction under a relatively high pressure, taking off in the gaseous state at least part of the nitrogen product of the rectification, and warming it up by heat exchange with a second air fraction under a relatively low pressure, expanding said first air fraction and partly liquefying it, warming up the unliquefied part of said first air frac' tion by heat exchange with said first air fraction, and thereafter evacuating it, feeding the liquefied part of said first air fraction at a constant rate into the rectification zone, and introducing said second air fraction at a constant rate and a constant cold content into said rectification zone.

5. A method for producing oxygen by the liquefaction and rectification of air and supplying it under pressure to a variable demand, comprising the steps of: drawing off the oxygen from the rectification zone, at least in part, in the liquid state and feeding it to a store of liquid oxygen, taking it from said store, compressing it in the liquid state and warming it up in heat exchange with a first air fraction under pressure, taking off in the gaseous state the remaining products of the rectification and warming them up in heat exchange with a second air fraction under pressure, expanding said first air fraction and liquefying it, reheating the unliquefied part of said first air fraction by heat exchange with said first air fraction, and thereafter evacuating it, feeding the liquefied part of said first air fraction at a constant rate into the rectification zone, and introducing said second air fraction at a constant rate and a constant cold content into the rectification zone.

6. A method for producing oxygen by the liquefaction and rectification of air according to a variable demand, comprising the steps of: taking off at least part of the oxygen product of rectification in the liquid state, at least partly warming it up in heat exchange with a first air fraction under pressure, taking oif in the gaseous state the remaining products of rectification and warming them up in heat exchange with a second air fraction under pressure, compressing air at a constant rate, cooling a variable portion thereof, constituting said first air fraction, in heat exchange with the liquid oxygen to be warmed up, expanding with external work the remaining portion of said compressed air, warming it up by heat exchange with said first air fraction and evacuating it, expanding said first air fraction and partly liquefying it, reheating the unliquefied part thereof by heat exchange with said first air fraction, and thereafter evacuating it, feeding the liquefied part of said first air fraction at a constant rate to the rectification zone, and introducing said second air fraction at a constant rate and a constant cold content into the rectification zone.

7. A method for producing oxygen by the liquefaction and rectification of air and delivering it to a variable demand, comprising taking oif at least part of the oxygen product of rectification in the liquid state and feeding it to a liquid oxygen store associated with the rectification zone, bringing it therefrom to at least a store associated with a remote oxygen consumption and air liquefaction zone, taking it from last said store and at least partly warming it up in heat exchange with a first air fraction under pressure, expanding said first air fraction and partly liquefying it, reheating the unliquefied part of said first air fraction by heat exchange with said first air fraction, and thereafter evacuating it, feeding the liquefied part of said first air fraction to a liquid air store associated with said oxygen consumption and air liquefaction zone, bringing it from said store to a store associated with said rectification zone, supplying it to said rectification zone at a constant rate, taking off in the gaseous state the remaining products of the rectification and warming them up by heat exchange with a second air fraction under pressure, and introducing said second air fraction at a constant rate and a constant cold content into said rectification zone.

8. A method for warming under pressure liquid oxygen and supplying it to a variable demand, and simultaneously liquefying air, comprising taking off liquid oxygen at a variable rate from a store, compressing it in the liquid state, warming it up in heat exchange with air under pressure, compressing air at a constant rate, cooling a first variable portion thereof in heat exchange with said compressed liquid oxygen, expanding it and partly liquefying it, feeding it to a liquid air store, evacuating from said store the unliquefied part of said portion and warming it up in heat exchange with said portion, expanding with external work the remaining portion of said air, warming it up in heat exchange with said first portion and evacuating it.

9. A method for warming under pressure liquid oxygen and supplying it to a consumer system having periods of no demand, periods of middle demand and periods of large demand, and simultaneously liquefying air, comprising: providing a store of liquid oxygen and a store of liquid air, compressing air at a constant rate, cooling it, expanding it and partly liquefying it, feeding the liquefied part thereof to said liquid air store and Warming up the unliquefied part thereof by heat exchange with said compressed air; during periods of no demand, expanding with external work a large part of the compressed air, while submitting to a free expansion the remainder of the compressed air; during periods of middle demand, expanding with external work a smaller part of the compressed air, while still submitting to said free expansion the remainder of the compressed air, drawing off some liquid oxygen from said liquid oxygen store, and warming it up by heating exchange with said compressed air; during periods of large demand, drawing ofi' a large amount of liquid oxygen from said liquid oxygen store, and warming it up by heat exchange with the compressed air, while submitting the whole of the compressed air to said free expansion.

10. Apparatus for supplying oxygen to a consumer system having a variable demand, comprising an air rectification system, a liquid oxygen container, piping means for introducing liquid oxygen from the oxygen reboiler of said rectification system into said liquid oxygen container, a liquid oxygen warming and air liquefying heat exchanger, piping means passing liquid oxygen from said container into said heat exchanger, a first air compressor, an air expander and valve and piping means associated therewith for passing part of the compressed air through said expander, heat exchange means between the compressed air and the air passed through the expander, a liquid air container, piping means and an expansion valve for feeding the compressed cooled air into said liquid air container, piping means and a control valve associated therewith for feeding liquid air from the liquid air container into said rectification system, piping means and heat exchange means for evacuating and warming up unliquefied air from said liquid air container, piping means for withdrawing the remaining rectification products from said rectification system, a second air compressor and piping means for introducing a compressed air stream into said rectification system, and heat exchange means between said remaining products of the rectification and said compressed air stream.

11. A liquid oxygen warming and air liquefying apparatus, adapted to receive a liquid oxygen supply from a distant air rectification apparatus and to produce gaseous oxygen for an oxygen consumer system having a variable demand, comprising a liquid oxygen container, a liquid oxygen warming and air liquefying heat exchanger, piping means and at least a liquid oxygen pump associated therewith for passing liquid oxygen from said container into said heat exchanger, an air compressor, at least an air expander and valve and piping means associated therewith for passing part of the compressed airthrough said expander, heat exchange means between the compressed air and the air passed through the expander, a liquid air container, piping means and an expansion valve for feeding the compressed cooled air into said liquid air container, and piping means and heat exchange means for evacuating and warming up the unliquefied air from said liquid air container.

12. A liquid oxygen warming and air liquefying apparatus according to claim 11, wherein the pip-ing means and liquid oxygen pumps between the liquid oxygen container and the liquid oxygen warming heat exchanger consist of at least two pipes and pumps in parallel, and the valve and piping means for passing compressed air to expanders consist of at least tWo lines in parallel, each provided with an expander.

References Cited in the file of this patent UNITED STATES PATENTS 2,503,939 De Bufire a- Apr. 11, 1950 2,626,510 Schilling Jan. 27, 1953 2,708,831 Wilkinson May 24, 1955 2,712,738 Wucherer July 12, 1955 2,741,094 Schuftan Apr. 10, 1956 2,822,675 Grenier Feb. 11, 1958 2,873,583 Potts Feb. 17, 1959 FOREIGN PATENTS 803,301 Germany Apr. 2, 1951 

